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  3. No very satisfactory account of the mechanism that caused the formation of t

No very satisfactory account of the mechanism that caused the formation of t

游客2025-02-25  7
问题     No very satisfactory account of the mechanism that caused the formation of the ocean basins has yet been given. The traditional view supposes that the upper mantle of the earth behaves as a liquid when it is subjected to small forces for long periods and that differences in temperature under oceans and continents are sufficient to produce con- vection in the mantle of the earth with rising convection currents under the mid-ocean ridges and sinking currents under the continents. Theoretically, this convection would carry the conti- nental plates along as though they were on a conveyor belt and would provide the forces needed to produce the split that occurs along the ridge. This view may be correct: it has the advantage that the currents are driven by temperature differences that themselves depend on the position of the continents. Such a back-coupling, in which the position of the moving plate has an impact on the forces that move it, could produce complicated and varying motions.
    On the other hand, the theory is implausible because convection does not normally occur along lines, and it cer- tainly does not occur along lines broken by frequent offsets or changes in direction, as the ridge is. Also it is difficult to see how the theory applies to the plate between the Mid-Atlantic Ridge and the ridge in the Indian Ocean. This plate is growing on both sides, and since there is no intermediate trench, the two ridges must be moving apart. It would be odd if the rising convection currents kept exact pace with them. An alternative theory is that the sinking part of the plate, which is denser than the hotter surrounding mantle, pulls the rest of the plate after it. Again it is difficult to see how this applies to the ridge in the South Atlantic, where neither the African nor the American plate has a sinking part.
    Another possibility is that the sink- ing plate cools the neighboring mantle and produces convection currents that move the plates. This last theory is attractive because it gives some hope of explaining the enclosed seas, such as the Sea of Japan. These seas have a typical oceanic floor, except that the floor is overlaid by several kilometers of sediment. Their floors have probably been sinking for long periods. It seems possi- ble that a sinking current of cooled mantle material on the upper side of the plate might be the cause of such deep basins. The enclosed seas are an important feature of the earth’s surface, and seriously require explanation because, in addition to the enclosed seas that are developing at present behind island arcs, there are a number of older ones of possibly similar origin, such as the Gulf of Mexico, the Black Sea, and perhaps the North Sea. [br] Which of the following, if it could be demonstrated, would most support the traditional view of ocean formation?
选项 A、Convection usually occurs along lines.
B、The upper mantle behaves as a dense solid.
C、Sedimentation occurs at a constant rate.
D、Sinking plates cool the mantle.
E、Island arcs surround enclosed seas.
答案 A
解析 下面哪一个如被指出，将最大程度支持传统的大洋构造观点?支持传统观念，即削弱作者观点。A．正确。“对流通常沿直线产生”。原文L27—29，对流通常不沿直线产生。作者据此推出传统观点不对。因此，传统观点若对，则原来反驳它的论据不能成立。即得出A选项的结论。B．上地幔表现为高密度固体。原文无。C．沉积以固定速率形成。无。D．下降板块使地幔变冷。此为第三观点内容，当然不能支持传统观点。E．岛屿链环绕内海，这是常识，不管哪种解释正确，它都成立。
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